Permanent Magnet Machine with Offset Pole Spacing

ABSTRACT

An internal permanent magnet machine has multiple rotor sections, each section having multiple rotor laminations. Permanent magnets are placed asymmetrically in lamination openings to attenuate oscillations in torque caused by harmonic components of magnetic flux.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an inner permanent magnet machine having arotor with multiple laminations in axially stacked relationship.

2. Background Art

An inner permanent magnet machine typically includes a stator withstator coil windings and a rotor with circumferentially spaced permanentmagnets on the rotor periphery that cooperate with circumferentiallyspaced stator poles separated from the periphery of the rotor with acalibrated air gap. When the machine is acting as a motor, the coils areenergized by an electrical current to effect rotation of the rotor. Thecurrent has an alternating waveform, typically of sinusoidal shape,which creates an electromotive rotor torque. The interaction of anelectromagnetic flux flow path created by the stator windings with theflux flow path created by the permanent magnets typically is accompaniedby harmonic waveform components that induce motor torque fluctuations.This is manifested by a motor torque ripple, or torque oscillation,accompanied by vibration and noise. Further, operating efficiency of themotor is affected adversely.

A conventional way to reduce motor torque ripple comprises skewingaxially placed sections of the rotor, one section with respect to theother. The rotor typically is connected drivably to a rotor shaft usinga keyway and slot driving connection. In order to offset or skew a rotorsection with respect to an adjacent section, the sections are relativelyrotated, usually about one-half of the stator slot pitch. If it isassumed that the rotor is divided into a given number of axial sections(k), the sections are rotated with respect to adjacent sections by anangle equal to:

skew angle(k)=360/(k×N _(S)) in mechanical degrees,

where N_(S) is

the number of slots.

The maximum rotation between any two axial sections of the rotor is:

max relative skew angle(k)=(k−1)×360/(k×N _(S))

in mechanical degrees.For example, in the case of a two section, 48 slot stator, a typicalvalue of the skew angle is 3.75°. The skewing of the rotor is intendedto produce a smoother mechanical torque than would otherwise be achievedusing a straight rotor. This will eliminate certain undesirableoscillations or ripple of the torque caused by harmonics present in theair gap flux and in the air gap permeance.

SUMMARY OF THE INVENTION

An objective of the invention is to minimize a so-called torque ripplewith minimal reduction in average torque. This differs from theinvention of copending application Ser. No. 11/839,928, filed Aug. 16,2007 entitled “Permanent Magnet Machine,” which is assigned to theassignee of the invention, in which an objective is to improve motorefficiency during operation in a motoring mode by using asymmetry inrotor design features of the motor while allowing an acceptable decreasein regenerative energy recovery during operation in a generating mode.

The present invention will break the symmetry of the rotor laminations,so that at a given instant the torque contributions of the multiplesections will be altered to reduce torque ripple.

In a first embodiment of the invention, torque ripple can be attenuatedby using radial skewing. This is done by offsetting the magnetic axis ofa rotor magnetic pole with respect to the axis of the adjacent rotormagnetic pole.

In a second embodiment of the invention, the rotor magnets are arrangedin a “V” configuration. The shape of the torque ripple is a function ofthe shape of the “V” configuration. By using at least two different,properly designed “V” configurations in the laminations, the totalmachine ripple can be reduced in amplitude.

In a third embodiment of the invention, the laminations in a multiplesection rotor are arranged in at least three rotor sections, which arerelatively rotated in small increments, one section with respect to theother. This can be done by using at least two pairs of key slotpositions. In this way, the axis of a magnetic pole of one section isplaced angularly with respect to the pole axis of the adjacent section.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a plan view of a rotor lamination;

FIG. 1 b is a side view of the rotor lamination; for the motor shown inFIG. 1 a;

FIG. 2 a is a diagrammatic view of a motor with a rotor comprised ofmultiple sections, each section being comprised of multiple laminationswherein flux lines are generated solely by the permanent magnet;

FIG. 2 b is a view similar to the view of FIG. 2 a wherein the statorhas energized windings with electrical current, but wherein the magnetsare not included, the flux lines being generated solely by the statorwindings;

FIG. 3 is a schematic representation of a prior art two-section rotor inwhich the sections are skewed, one with respect to the other, accordingto a known skewing technique;

FIG. 4 shows a symmetric lamination for a prior art eight pole rotordesign for use in the electric motor rotor seen in FIG. 3;

FIG. 5 shows a rotor lamination with a radial skewing in accordance witha first embodiment of the present invention;

FIG. 6 is a plot of rotor rotation angle in mechanical degrees versusmotor torque in Newton meters showing the effect on motor instantaneoustorque using the magnet arrangement of FIG. 5;

FIG. 7 is a view of a continuous skewing arrangement that may be usedrather than the skewing arrangement of FIG. 5;

FIG. 8 is an illustration of a magnet distribution that has a moreuniform separation between the rotor poles compared to the rotorseparation of FIG. 7;

FIG. 9 is a plot of motor torque versus rotation angle in mechanicaldegrees for the rotor design illustrated in FIG. 8;

FIG. 10 shows a prior art rotor configuration of a permanent magnetmotor together with some of the variables that can be used to manipulatethe harmonic content of the motor torque;

FIG. 11 is a view of a portion of a laminated rotor with two angularpositions of the magnets for adjacent rotor sections;

FIG. 12 shows a view of a laminated rotor in which adjacent magnets arearranged with a different angle theta at alternate rotor locations;

FIG. 13 is a schematic representation in three-dimensional form showingthe axial alignment of the magnetic poles in a four pole structure;

FIG. 14 shows a prior art skewing of rotor laminations of a permanentmagnet rotor of the type shown in FIG. 3;

FIG. 15 shows the effect of flipping a second section of a rotor withrespect to a first section;

FIG. 16 is an illustration of a prior art final skewing technique afteradjacent sections of the rotor have been aligned along a key slot forthe sections shown in FIGS. 14 and 15;

FIG. 17 is an illustration of an embodiment of the invention wherein twokey slots are placed relative to each other at approximately 90° toallow construction of a four-section rotor;

FIG. 18 is a view similar to FIG. 17, but which illustrates the firsttwo sections of a four section rotor;

FIG. 19 is a combined view of the rotor sections of FIGS. 17 and 18;

FIG. 20 is an illustration of the final assembly of the four sections ofFIGS. 17-19;

FIG. 21 is an enlarged view of the final assembly of the four sectionsseen in FIG. 20;

FIG. 22 is a view similar to the views of FIGS. 18-21 with key slots toimprove balancing;

FIG. 23 is a final assembly view of the rotor laminations shown in FIG.22;

FIG. 24 is a view of a rotor design seen in FIG. 23, but which isprovided with built-in keys instead of key slots;

FIG. 25 is a plot showing a reduction in torque ripple for aconventional skewed design for the present invention and for a rotorthat is unskewed; and

FIGS. 26, 27 and 28 show examples of hybrid electric vehicle powertrainarchitectures capable of using the motor of the present invention.

PARTICULAR DESCRIPTION OF EMBODIMENTS OF THE INVENTION

For the purpose of describing typical operating environments for thepermanent magnet machine of the invention, reference first will be madeto FIGS. 26, 27 and 28, which respectively illustrate a power-splithybrid electric vehicle powertrain, a detailed power-split hybridelectric vehicle powertrain corresponding to the powertrain of FIG. 26and a series hybrid electric vehicle powertrain. In the case of thepowertrain schematically illustrated in FIG. 28, an engine 10 ismechanically connected to a generator 12, which in turn is electricallycoupled to an electric motor 14. Typically, the electrical couplingincludes a DC link comprising an AC/DC inverter 16 and a DC/AC inverter16′. A high-voltage traction battery 18 is coupled to the DC linkthrough a DC/DC converter 20. The motor is mechanically coupled to ageared transmission mechanism 22, which may have multiple-ratio gearingor single-ratio gearing.

Traction wheels 24 are driven by torque output elements of thetransmission mechanism. All of the mechanical energy of the engine,except for power losses, is transferred to the generator, which convertsmechanical energy to electrical energy for driving the motor 14. Anyelectrical energy not required to drive the motor is used to charge thebattery 18. When the vehicle is braking, all or part of the vehiclemechanical kinetic energy transferred from the transmission to the motor14, except for losses, is used to charge the battery as the motor 14acts as a generator.

In contrast to the series arrangement of FIG. 28, the series-parallelarrangement of FIG. 26 includes a direct mechanical connection betweenthe engine and the transmission, as shown at 26. The series-parallelgearing of the hybrid powertrain of FIG. 26 is shown in more detail inFIG. 27. Components that are counterparts for components in the seriesarrangement of FIG. 28 have been indicated by common reference numerals,although prime notations are added to the numerals in FIGS. 26 and 27.

The mechanical connection between the transmission 22′ and the engine10′ includes a planetary gear system 26. The planetary gear system, seenin FIG. 27, includes a ring gear 28, which acts as a power output memberfor driving a power input element of the transmission mechanism 22′. Asun gear 30 is mechanically connected to generator 12′. The carrier forthe planetary gear unit 26, shown at 32, is connected to the poweroutput shaft or crankshaft of the engine 10′. As the engine deliverstorque through the planetary gear unit 26, to the transmission. The sungear acts as a reaction element since it is mechanically connected tothe generator. The load on the generator thus will determine the speedof the engine. During forward drive, torque of motor 14′ complementsengine torque and provides a second power input to the transmission.During reverse drive, the torque direction of the motor 14′ is changedso that it will operate in a reverse direction. The engine is inactiveat this time.

When the vehicle is in a braking mode, regenerative energy is deliveredfrom the wheels through the transmission to the motor. The motor at thistime acts as a generator to charge the battery. A portion of theregenerative energy is distributed through the transmission to theengine in a mechanical torque flow path, shown in part at 26′ in FIG.26. In this respect, the regenerative energy flow path of the powertrainof FIG. 26 differs from the energy flow path for the powertrain of FIG.28, where no mechanical energy during regenerative braking isdistributed to the engine.

The rotor and the stator for the disclosed embodiments of the inventionmay be comprised of ferrous alloy laminations. A rotor and statorconstruction of this type is shown in the partial radial cross-sectionalview of FIG. 1 b. A stator lamination is shown at 36 in FIGS. 2 a and 2b, and a rotor lamination is shown at 38. A small air gap 40, seen inFIGS. 1 c and 2, is located between the inner periphery of the statorlaminations 36 and the outer periphery of the rotor laminations 38.Radially extending openings 37 are formed in the stator laminations andsymmetrically positioned magnet openings 42 are formed near the outerperiphery of each rotor lamination 38. Each magnet opening receives amagnet 44. Any number of laminations in a given design may be used,depending on design choice. The laminations are arranged in a stack.Multiple stacks (e.g., one, two or three) may be used.

FIG. 1 a and FIG. 1 b illustrate a rotor section construction withmultiple laminations arranged in stacked relationship. The magnetopenings are shown in FIG. 1 a, but this figure omits an illustration ofthe magnets.

The center of the rotor laminations has a circular central opening 60for accommodating a driveshaft with a keyway that may receive a drivekey 62.

The openings 42 are symmetrically disposed with respect to adjacentpairs of magnet openings 42, one of the axes of symmetry being shown inFIG. 1 a.

FIG. 2 a is a partial view of a rotor lamination 38. The stator 36 hasstator windings in the openings 37, but they are not illustrated in FIG.2 a because it is assumed that in the case of FIG. 2 a, the statorwindings do not carry electrical current. The stator windings withcurrent, however, are shown in FIG. 2 b.

A magnetic rotor flux flow path is shown at 65 in FIG. 2 a. A magneticstator flux flow path is shown at 65 and 66 in FIG. 2 b. The rotor fluxand the stator flux interact, as shown in part at 68, to develop rotortorque in known fashion.

A known way to reduce motor torque ripple is to skew the sections of therotor, one with respect to the other, by offsetting one half of therotor lamination stack with respect to the other half. This is seen inFIG. 3, where the X-axis 90 for rotor section 92 is skewed relative tothe Y-axis shown at 94 for an adjacent rotor section 96. The amount ofrotation of one section relative to the other is usually one half of thestator key or slot pitch. This is expressed as follows:

skew angle=180°/N _(S) in mechanical degrees, where N _(S) is the numberof slots.

Magnet openings in rotor section 92 are shown at 98. The magnet openingsare evenly spaced in the case of the rotor of FIG. 3. Magnet openingssimilar to openings 98 are located in rotor section 96. The rotorspacing about the Z-axis 102 in FIG. 3 is uniform. Reference may be madeto U.S. Pat. No. 7,170,209 for an illustration of a motor rotor withskewed rotor sections.

Magnet openings in the rotors of the embodiments of the invention thatare disclosed need not be shaped as shown in the figures of thedrawings. The shape of the magnet openings is a design choice.

FIG. 4 shows a plan view of a typical lamination for the sectionsillustrated in FIG. 5. As in the case of FIG. 2, rotor sections havinglaminations of the type shown in FIG. 4 may include a key-and-slotconnection with a rotor driveshaft, although the key-and-slot connectionis not shown in FIG. 4.

The rotor design having sections, as illustrated in FIG. 4, is dividedinto a generic number of axial sections K, each section being rotatedwith respect to an adjacent section by an angle equal to:

skew angle(k)=360/(k*N _(S)) in mechanical degrees, where N _(S) is thenumber of slots,

The maximum rotation between any two axial sections of the rotor is:

max relative skew angle(k)=(k−1)*360/(k*N _(S)) in mechanical degrees.

The magnet poles are located as shown in FIG. 4. The angle between themagnetic axes of adjacent poles is 45° for an eight pole design. Theangle between the magnetic axes and the interpolar axes is one half ofthe angle between the magnetic axes of adjacent poles for an eight poledesign.

The disclosed embodiments of the invention have eight magnetic poles,but the scope of the invention is not limited to the use of eightmagnetic poles. The number of poles used is a matter of design choice.

A first embodiment of the invention is shown in FIG. 5, where a rotorlamination has poles that are radially skewed. The skewing is realizedwithin each lamination itself by offsetting the magnetic axis of a motorpole with respect to an adjacent pole.

Manufacture of the rotor is simplified by the absence of several stepsusually needed to create multiple, axially-stacked rotor sections. Thismanufacturing method is especially valuable in the case of an integratedstarter-generator type motor, where the stacked length of the sectionsis normally short and the known skewing method described with referenceto FIG. 3 is not feasible. The embodiment of the invention, however, isnot limited to short stack motors and generators, but it can be appliedto any permanent magnet machine. It can exceed the performance of anelectric machine with known skewing and it may be made using simplermanufacturing processes. The performance improvement is due to a furtherreduction of the torque ripple previously described. Further, theembodiment of the invention of FIG. 5 is not limited by the number ofaxial segments in the rotor design. It has as many pole-spacingpossibilities as the number of rotor poles.

In the design of FIG. 5, the spacing between the axis of symmetry of twoadjacent magnets is not constant. It can be either one of two values:

i.e., Alpha₁=360/poles+skew angle;

or

Alpha₂=360/poles−skew angle.

For an eight pole, 48-slot motor and a skew angle of 3.75°, Alpha1 andAlpha2 are 48.75 and 41.25 mechanical degrees, respectively. Othervalues of skew angle can be chosen according to design choice. Theeffect of this magnet arrangement on the motor torque for a typicalinner permanent magnet machine is shown in FIG. 6. A typical rotortorque ripple plot for a non-skewed rotor is shown at 110 in FIG. 6 anda corresponding rotor torque ripple plot for a skewed rotor, accordingto the invention, is shown at 112. The amplitude of the ripple of plot112 is significantly lower than the amplitude of plot 110.

This rotor design is also suitable for other arrangements for the rotorpoles, such as the one shown in FIG. 7, where poles 1-8 are separated byan angle alpha=45+skew angle/7, and poles 8 and 1 are separated by anglebeta=45−skew angle. In contrast, for the design shown in FIG. 5, theskew angle is arbitrarily set to be equal to 3.5°, alpha=45.5° andbeta=41.5°.

An effect on torque ripple, similar to the effect on torque ripple forthe design of FIG. 5, can be obtained by the distribution pattern forthe magnets seen in FIG. 8. In FIG. 8, for any given pole, the offsetwith respect to the original magnetic axis remains the same as the oneshown in FIG. 7 (i.e., pole number 2 has a magnetic axis that isdisplaced 22.00° from one interpolar axis and 23° from the adjacentinterpolar axis), but pole number 3 has taken the place of pole number8, and pole number 4 has been moved to the location of pole number 3,etc. This distribution has a more uniform spacing between the poles thanin the case of the design of FIG. 7.

A plot of the motor torque versus rotation angle for the design of FIG.8 is seen in FIG. 9. The torque ripple seen in FIG. 9 is identified bynumeral 106. For purposes of comparison, the torque ripple for a rotorhaving sections using the known design with no skew is shown at 108.

A plot of motor torque versus rotation angle for the design of FIG. 5,as previously mentioned, is seen in FIG. 6 where a conventional designwith no skew is plotted at 110 and the plot corresponding to the designof FIG. 5 is shown at 112. The amplitude of the ripple seen at 106 inFIG. 9 has a lower amplitude than the amplitude seen at 112 in FIG. 6for the design of FIG. 5.

The embodiment of FIGS. 7-10 is not limited to flat magnets. It may have“V” shape magnets or other shapes.

FIG. 10 shows a magnet configuration in which the rotor magnets, seen at114 and 116, are arranged in a “V” shape. In the case of the design ofFIG. 10, the shape and the amplitude of the torque ripple is a functionof the shape and amplitude of the angle theta between the magnets 114and 116. Parameters that affect this shape and the magnitude of each areidentified in FIG. 10, where the width of each magnet may be 19.25 mmand the distance between a point of engagement of the magnets 114 and116 and the air gap may be 10.75 mm. The specific parameters, of course,can be different than those illustrated in FIG. 10.

FIG. 11 shows how the angle theta is adjusted to obtain smoother torqueproduction. Although the average values for the torque will not begreatly affected, the harmonic components of the torque can bemanipulated by properly designing the different “V” shapes. For purposesof illustration, magnets 118 and 120 for laminations of one section areshown overlapped with respect to magnets for laminations of an adjacentsection. Magnets 118 and 120 for one section are separated by an angletheta₁, whereas the angle for an adjacent section is theta₂.

In addition to the implementation of the invention seen in FIG. 11, themultiple magnetic poles on the rotor can be designed with at least twodifferent arrangements. For example, the eight pole rotor of FIG. 12 mayhave poles 1, 3, 5, and 7 of laminations of one section arrangedaccording to the design of FIG. 11, in which the angle is theta₁, andthe other four poles may have a design in which the angle is theta₂.Further, to avoid low frequency torque oscillations, the rotor can bedivided into two axial segments for the design shown in FIG. 11, whichare rotated with respect to each other, so that poles 1, 3, 5 and 7 ofone section of the rotor are aligned with poles 2, 4, 6 and 8 of anadjacent section. This arrangement is shown in FIG. 13, where themagnetic axis of a set of poles A for one section is aligned with theaxis of a set of magnetic poles B for an adjacent section.

The concept illustrated in FIG. 13 can be extended to include rotorconfigurations that are different from the “V” shaped configuration seenin FIGS. 11 and 12. For example, pole type A in FIG. 13 could include“V” shaped magnets and pole type B could be flat or surface mountedmagnets, as in the case of FIGS. 5, 7 and 8. The torque harmonics can bemanipulated in this fashion to create an attenuated total torque ripple.Further, more than two types of magnet configurations can be used, andvariations can be made in the proximity of the magnets to the air gap tomanipulate torque harmonics.

A third embodiment of the invention makes it possible to form thelaminations of multiple axial sections in a manufacturing process usinga single rotor lamination stamping die in order to avoid multiplelamination types in the same rotor.

FIG. 14 shows a known skewing arrangement for a permanent magnet motorusing a single lamination type where the first section of the rotor isassembled by stacking half of the rotor laminations and inserting themagnets in their magnet openings. The laminations have a key slot 124,where key slot axis 130 is rotated with respect to the nearest pole axis126 by a certain angle. To create the second section of the rotor, therest of the laminations are flipped around axis 130, as shown in FIGS.14 and 15. Because of the flip, the pole axis shown at 126 in FIG. 14becomes pole axis 128 in FIG. 15 and is rotated by Gamma with respect toaxis 130 in the counter-clockwise direction. The optimum angle would bedetermined based on the harmonic content of the air gap flux and the airgap permeance. When the two rotor sections are aligned using the keyslot as the common aligning device, the pole axes of the two rotorsections are displaced by 2× Gamma with respect to each other.

Performance of the rotor shown in FIG. 16 can be improved to approximatea continuous skewing effect by increasing the number of rotor sectionsand rotating them in smaller incremental steps. The present invention isaimed at accomplishing this task using a single lamination die. This isillustrated in FIG. 17, which shows laminations with a first key slot at132 and a second key slot at 134. The key axis for slot 132 is shown at136, and the key axis for slot 134 is shown at 138.

The first section of the rotor is obtained by axial stacking one quarterof the rotor laminations and then aligning them along the first keyslot. The second section is similarly made by flipping the laminationsand stacking them, as in the design of FIGS. 14-16. This will result inthe partial assembly shown in FIG. 18, which illustrates the first twostages of a four section rotor. The angle formed by 136 (Gamma1) and thenearest pole axis 142 is different from the angle 138 (Gamma2) and thenearest pole axis 144. The magnetic axes shown at 140 and 142 in FIG. 18in one example of the invention are separated by 2× Gamma1. The key slotaxis is shown at 136 for the slot 132.

The included angle created by the intersection of axes 136 and 138 forkey slots 132 and 134, respectively, may be referred to as angle Delta,expressed as:

Delta=N*360/P+2Gamma1, where P is the number of poles, and N is anynumber in the number set of 1, 2, 3, . . . P−1.

In FIG. 19, the third section of the rotor assembly for the thirdembodiment of the invention is created from the non-flipped laminations,as in the case of the first section, but it is rotated to the angleDelta clockwise so that it is aligned with the first two sections usingthe second key slot 134. In FIG. 19, numeral 154 designates the key slotaxis. The third section in the design of FIG. 19 is offset bytheta2−theta1 from the first section. FIG. 20 shows the final assemblyof all four sections.

FIG. 21 is an enlargement of a portion of the four sections illustratedin FIG. 20. It shows the axis of the magnetic poles of the differentsections relative to the key slot axes.

The fourth section of FIGS. 20 and 21 is created from flippedlaminations, which are rotated counter-clockwise by the angle Delta andaligned on the second key slot. The resulting structure shown in FIGS.20 and 21 has four sections, which have the following rotations withrespect to the shaft key: The first is shown at 158, which is rotatedtheta1 in a counter-clockwise direction; the second section shown at 160is rotated theta1 in a clockwise direction; the third section shown at162 is rotated theta2 in a counter-clockwise direction; and the fourthsection shown at 164 is rotated theta2 in a clockwise direction. The keyslot axis is shown at 166.

It is possible with this embodiment of the invention to arrange thelaminations so that the second key slot is aligned with the magnet axis.In this case, the third and fourth rotor sections will have zerorotation and a balanced symmetrical three section rotor thus becomespossible.

Rotor balancing can be improved in the design of the third embodimentwith the adoption of a second set of key slots placed at 180° from theother two, as illustrated in FIG. 22, where key slots 168 and 170 arespaced 180°, respectively, from key slots 172 and 174. Following theprocedure described previously with respect to FIG. 21, the resultingdesign is illustrated in FIG. 23 where two keys are used to secure therotor to the rotor shaft to achieve improved rotor balance. This is seenat 176 and 178 in FIG. 23.

It is possible in the case of the configuration according to the thirdembodiment of the invention to use a pole number count other than a polecount of eight. A four pole rotor can be treated in the same way as arotor with a pole count of eight poles. Also, the assembly technique canbe applied to rotors that do not use a key slot, but rather use a tab orother alignment device, such as a cleat. Further, built-in keys can beused as illustrated in FIG. 24. In the case of the design of FIG. 24,the rotor shaft will have two key slots 180 and 182 that are as wide asthe keys, and two larger key slots 184 and 186 that accommodate amisalignment of the keys, shown at 188 and 190, respectively.

FIG. 25 is a plot of torque ripple obtained by the embodiments of theinvention using a finite element simulation technique. A conventionalmethod of skewing will result in a ripple plot as shown at 192. A plotusing three lamination sections according to the present invention isshown at 194. For purposes of comparison, an unskewed rotor plot isshown at 196.

Although embodiments of the invention have been disclosed, it will beapparent to persons skilled in the art that modifications may be madewithout departing from the scope of the invention.

1. A permanent magnet electric machine comprising: a stator withelectromagnetic windings for stator poles; a rotor located coaxiallywith the stator and having an air gap therebetween; and a plurality ofpermanent magnets positioned on the rotor periphery so that magneticpoles of adjacent magnets are asymmetrical, whereby harmonic componentsof air gap magnetic flux are attenuated to reduce undesirableoscillation of rotor torque.
 2. A permanent magnet machine comprising astator with electromagnetic windings for stator poles and a rotorlocated on a rotor axis that is common to an axis for the stator with anair gap between the stator and the rotor; the rotor having a pluralityof permanent magnets on its periphery; and magnetic poles for themagnets being characterized by magnetic flux flow patterns that interactwith flux flow paths for the electromagnetic stator windings to createrotor torque; the magnetic poles being asymmetrically positioned on therotor periphery whereby harmonic components of air gap flux areattenuated to reduce undesirable oscillation of rotor torque.
 3. Thepermanent magnet machine set forth in claim 2 wherein the magnets arepositioned on the rotor to effect radial skewing, the poles of at leastone pair of adjacent magnets being spaced in closer proximity than polesof other magnet pairs on the rotor periphery whereby asymmetry isachieved.
 4. The permanent magnet machine set forth in claim 3 whereinpairs of magnets define rotor poles with a rotor pole axis that arenon-uniformly spaced about the rotor periphery with respect to rotorpole axes defined by other adjacent pairs of magnets.
 5. The permanentmagnet machine set forth in claim 3 wherein the rotor pole axes areuniformly spaced about the rotor periphery.
 6. A permanent magnetmachine comprising a stator with electromagnetic windings for statorpoles and a rotor located on a rotor axis that is common to an axis forthe stator with an air gap between the stator and the rotor; the rotorhaving a plurality of pairs of permanent magnets on its periphery, themagnets of each pair being located in a “V” shape configuration definingan angle therebetween; the rotor comprising laminations arranged inmultiple sections in stacked axial relationship; magnetic poles for themagnet pairs being characterized by flux flow patterns that interactwith flux flow paths for the electromagnetic stator windings to createrotor torque; the magnets of one pair for one section defining an angletherebetween that differs from an angle defined by the magnets of anadjacent rotor section whereby harmonic components of rotor torque aremanipulated to obtain smooth torque production.
 7. The permanent magnetmachine set forth in claim 6 wherein the magnets have pre-calculatedlengths and widths, the width of the magnets in one section beingdifferent than a corresponding width of magnets of another sectionwhereby rotor torque fluctuations are modified.
 8. The permanent magnetmachine set forth in claim 6 wherein the shape of the magnets in onesection are different than the shape of the magnets in another sectionwhereby motor torque fluctuations are modified.
 9. The permanentmagnetic machine set forth in claim 6 wherein the length of magnets inone section are different than the length of magnets of another sectionwhereby motor torque fluctuations are modified.
 10. The permanent magnetmachine set forth in claim 6 wherein the magnets of each pair define amagnetic axis; the rotor having at least two rotor sections arranged inaxially-stacked relationship, each rotor section having a plurality oflaminations in stacked assembled relationship; the magnetic pole axis ofone rotor section being in axial alignment with the magnetic pole axisof an adjacent rotor section having a different magnet arrangement. 11.A permanent magnet machine comprising a stator with electromagneticwindings for stator poles and a rotor located on a rotor axis that iscommon to an axis for the stator with an air gap between the stator andthe rotor; the rotor having a plurality of pairs of permanent magnets onits periphery, the magnets of each pair being located in a “V” shapeconfiguration defining an angle therebetween; the rotor comprisinglaminations arranged in multiple sections in stacked axial relationship;magnetic poles for the magnet pairs being characterized by flux flowpatterns that interact with flux flow paths for the electromagneticstator windings to create rotor torque; the magnets of one pair for onesection defining an angle therebetween that differs from an angledefined by another pair of magnets of the same rotor section wherebyharmonic components of rotor torque are manipulated to obtain smoothtorque production.
 12. The permanent magnet machine sets forth in claim11 wherein the magnets have pre-calculated lengths and widths, the widthof the magnets in one section being different than a corresponding widthof magnets of another section whereby rotor torque fluctuations aremodified.
 13. The permanent magnet machine set forth in claim 11 whereinthe length of magnets in one section are different than the length ofmagnets of another section whereby rotor torque fluctuations aremodified.
 14. The permanent magnet machine set forth in claim 11 whereinthe magnets of each pair define a magnetic axis; the rotor having atleast two rotor sections arranged in axially-stacked relationship, eachrotor section having a plurality of laminations in stacked assembledrelationship; the magnetic axis of one rotor section being in axialalignment with the magnetic axis of an adjacent rotor section having adifferent magnet arrangement.
 15. A permanent magnet machine comprisinga stator with electromagnetic windings for stator poles and a rotorlocated on a rotor axis that is common to an axis for the stator with anair gap between the stator and the rotor; the rotor comprising aplurality of axially-stacked sections, each section comprising aplurality of stacked laminations on a rotor shaft with a key and slotdriving connection therebetween; each section having a plurality ofmagnets located on its periphery, the magnets creating a magnetic fluxpattern that interacts with a magnetic flux pattern created by themagnetic windings of the stator poles to develop rotor torque; thelaminations in one section having asymmetrically placed first magnetopenings for receiving rotor magnets; the laminations of a secondsection having asymmetrically placed second magnet openings forreceiving rotor magnets; the magnet openings for the laminations of thefirst section having a common geometry with respect to the magnetopenings for the laminations of the second section to permit manufacturewith a common machine tool; the laminations for the second section beingflipped about a radial axis relative to the laminations for the firstsection following assembly of the rotor, the rotor openings for thelaminations of the second section being angularly displaced relative tothe rotor openings for the laminations of the first section, wherebyrotor torque fluctuations are attenuated.
 16. The permanent magnetmachine set forth in claim 15 wherein the rotor comprises more than tworotor sections, the laminations for the second section being flippedabout a first radial axis relative to the laminations of the first rotorsection; and the laminations for a third rotor section being flippedabout a second radial axis relative to the laminations of the firstrotor section, whereby rotor torque fluctuations are attenuated.
 17. Thepermanent magnet machine set forth in claim 16 whereby the first andsecond radial axes are displaced to an angle Delta.
 18. The permanentmagnet machine set forth in claim 15 whereby the sections are displacedangularly, one with respect to the other, about a geometric rotary axisof the rotor during assembly of the rotor sections.
 19. The permanentmagnet machine set forth in claim 15 wherein the laminations of onesection have driving key and slot features at locations spacedapproximately 180° apart about a geometric rotary axis of the rotor. 20.The permanent magnet machine as set forth in claim 16 wherein thelaminations of the first section has first driving key and slot featuresat locations spaced approximately 180° apart about a geometric rotaryaxis of the rotor; the laminations of the second section having seconddriving key and slot features at locations spaced approximately 180°apart about the geometric rotary axis of the rotor; the first drivingkey and slot features being relatively displaced approximately 90° fromthe second driving key and slot features.
 21. The permanent magnetmachine set forth in claim 20 wherein the permanent magnet machine hasthird and fourth lamination sections with driving key and slot featuresrelatively displaced to an angle Delta from the second driving key andslot features of the laminations of the first and second sections. 22.The permanent magnet machine set forth in claim 21 wherein thelaminations of the third and fourth sections are flipped about radialaxes extending through the first and second key and slot features. 23.The permanent magnet machine set forth in claim 20 wherein the key andslot features are configured to provide axial alignment of thelamination sections in axially stacked relationship.
 24. The permanentmagnet machine set forth in claim 21 wherein the key and slot featuresare configured to provide axial alignment of the lamination sections inaxially stacked relationship.
 25. The permanent magnet machine set forthin claim 22 wherein the key and slot features are configured to provideaxial alignment of the lamination sections in axially stackedrelationship.